Once defeated, the giant decides to convert to Christianity and joins Charlemagne's army, but Olivier and several other knights are captured. Floripas, Fierabras' sister, falls in love with one of Charlemagne's knights, Gui de Bourgogne. After a series of adventures, Charlemagne kills king Balan, divides Spain between Fierabras and Gui de Bourgogne (who marries Floripas), and returns to Saint Denis with the holy relics.

The poem also survives in an Occitan version dating from the 13th century (roughly 5,000 alexandrines; the first 600 verses do not appear in the Old French version).[1] The Occitan and the Old French version may derive from a common lost source.[2] This version in turn inspired an Italian version (Cantare di Fierabraccia e Ulivieri) in the second half of the 14th century.[3]

Two English versions were made: Sir Ferumbras (late 14th or early 15th century) and Firumbras (fragmentary).[3] A 15th-century English work, Sowdon of Babylon, combined the story with another work (the Destruction de Rome).[3]

The story was put into prose three times in the 14th and 15th centuries:

one anonymous version (14th century);[3] in this version, among the various changes brought to the story, Fierabras is no longer depicted as a giant.[3]

a Burgundian version (expanded with other material from the Matter of France and the history of Charlemagne: Chroniques et conquêtes de Charlemagne) by David Aubert (c. 1456–8)[3]

and, most importantly, a Swiss version by Jean or Jehan Bagnyon,[3]Le rommant de Fierabras le geant (Geneva, 1478,[4] the first chanson de geste to be printed[1]) which the author (like David Aubert) expanded with other material from the Matter of France and the history of Charlemagne (from 1497 [1] the title was La Conqueste du grand roy Charlemagne des Espagnes et les vaillances des douze pairs de France, et aussi celles de Fierabras[4]). The historical material in Bagnyon's text is largely based on the Historia Caroli Magni (also known as the "Pseudo-Turpin" chronicle), probably known to Bagnyon via the Speculum Historiale of Vincent de Beauvais.[5] The Bagnyon version became one of the most popular novels in France in the first half of the 16th century (15 editions printed to 1536[4]) and was adapted into Castilian, Portuguese, German, and English (by William Caxton).[3]

In Spain the story can be found in the Historia del emperador Carlomagno y de los doce pares de Francia by Nicolás of Piemonte first edited in 1521. This is a Castilian translation—or better, an adaptation—of Bagnyon's La Conqueste du grand roy Charlemagne. Miguel de Cervantes refers to Fierabras in his Don Quixote (see below).

There also exist other versions of the legend, including one in Early Modern Irish (Stair Fortibrais).[3]

The 17th-century playwright Calderón de la Barca used elements of the story (the love affair of Floripas and Gui) for his play La Puente de Mantibile.

The story echoes the historical Arab raid against Rome in 846 in which Guy I of Spoleto (proposed as a source for "Gui de Bourgogne") participated,[6] and critics have suggested that the existing "chanson" was based on a now lost poem describing the Sack of the Roman Basilicas extra muros.[1]

The composition of the 12th-century poem may be closely linked to the cult of relics at the Basilica of St Denis in Paris and the creation of the local festival of Lendit,[7] as the narrator in the Old French poem addresses himself to visitors at this fair.[1]

Another view is that the Legend is based on the character of the Basque prince, Fortun "the Basque" Al-Graseiz or El-Akraz, as seen by the Arab chroniclers and perhaps known as such by Shakespeare to bring it over to his exotic character Fortinbras.

This is the tale that Robert the Bruce, King of Scots, is said by Barbour to have related to his men after they fled their enemies across Loch Lomond in 1307.[8]